Generally, a hydraulically driven variable-displacement traveling motor is used as a traveling control apparatus for a hydraulic traveling vehicle, such as a wheel-mounted hydraulic excavator. If the loading pressure (travel loading pressure) of the traveling motor increases in the acceleration state, slope-ascending state, or the like, the traveling motor displacement is increased to generate required torque. In the deceleration state, an over-load relief valve provided in a hydraulic drive circuit for traveling controls the circuit pressure (brake pressure) and generates braking force in the traveling motor by means of the brake pressure to brake the vehicle body. With a thus-configured traveling control apparatus, if a traveling-directional vector of a vehicle body weight component with respect to the gradient exceeds the above-mentioned braking force during slope-descending operation, the speed of the vehicle body is increased by its weight, and a predetermined maximum vehicle body speed is exceeded, affecting the life of equipment used.
A traveling control apparatus described in Patent document 1, in a state where the traveling-directional vector of a vehicle body weight component with respect to the gradient is likely to exceed the above-mentioned braking force during slope-descending operation, detects the neutrality of the traveling pedal and increases the traveling motor displacement by use of an external command to increase the braking force, thus preventing the maximum speed during slope-descending operation from exceeding a predetermined speed.
A traveling control apparatus described in Patent document 2 supervises the vehicle speed in advance and, if the vehicle traveling speed exceeds a predetermined speed, increases the traveling motor displacement by use of an external command, thus preventing the maximum high speed during slope-descending operation from exceeding a predetermined speed.
On the other hand, some traveling control apparatuses for a hydraulic traveling vehicle, such as a wheel-mounted hydraulic excavator, enable setup of the high-horsepower operation mode. Such a traveling control apparatus makes it possible to set the high-horse power operation mode as required to increase the maximum engine speed to ensure high horsepower. Such a traveling control apparatus performs various control operations to prevent vehicle speed increase when the high-horsepower operation mode is set. For example, a conventional art described in Patent document 3 decreases the tilting (displacement) of the hydraulic pump to prevent vehicle speed increase when the high-horsepower operation mode is set. The art described in Patent document 3 provides a self-pressure tilting control mechanism for increasing the traveling motor displacement by the self-pressure of the traveling motor if the delivery pressure of the hydraulic pump becomes higher than a predetermined pressure. The art detects the delivery pressure of the hydraulic pump, increases the maximum engine speed if the detected delivery pressure becomes higher than a predetermined pressure, thus preventing vehicle speed increase when the high-horsepower operation mode is selected (when the maximum engine speed increases).
Patent document 1: JP-A-8-270788
Patent document 2: Japanese Patent No. 3631620
Patent document 3: JP-A-2001-295682